This invention relates generally to a method of high resolution electrostatic transfer of a high density image to a receiving surface. More specifically, it pertains to the use of a dry powder electrophotographic toner with a permanent master made from a photopolymer material to transfer a developed image via a reversal photographic process from the master to the receiving substrate. The permanent master may be repeatedly used to produce high resolution and high density images on receiving surfaces, such as printed circuit boards, with the dry powder toner.
The production of conductive wiring patterns on an insulating substrate employing a dry film resist by use of photoimaging and other techniques to produce a printed circuit board typically employs a five step process. Regardless of whether a tenting method or a hole-plugging method is employed, the five distinct steps have included laminating or coating a photopolymer dry film resist on at least one conductive surface of an insulating substrate, forming a wiring pattern on the dry film resist by use of artwork or a phototool and exposing the dry film resist to actinic radiation through the transparent areas of the phototool, developing the circuit board by removing the unexposed portions of the negative working dry film resist, etching the conductive substrate from the circuit board in all non-imaged areas not beneath the desired conductive wiring pattern which is still covered with the dry film resist, and finally stripping or removing the dry film resist covering the desired wiring pattern from the non-etched portions of the conductive substrate. This five step process must be repeated for each circuit board produced.
During the exposure step in the standard dry film process, sufficient radiation exposure levels and exposure times are desired to produce straight sidewalls in the dry film resist that are the result of a pattern of the cross-linking of polymers in the dry film. These straight sidewalls should be normal to the conductor surface. Practically, however, for example in the standard negative working dry film photoresist print and etch process, either underexposure occurs, producing a sidewall edge that undercuts the desired resist pattern, or overexposure occurs, producing a sidewall edge in the dry film photoresist that increases the width of the dry film photoresist at the base of the resist and the surface of the conductor causing a foot. Both of these conditions vary the width of the ultimate conductive pattern from that which is desired, beyond the planned and engineered tolerance or overage of the line widths in the conductor surface.
The development step during this process ideally should dissolve away the unexposed and, therefore, uncross-linked areas of the negative working dry film resist to produce an edge in the dry film resist on the conductor surface that is equal in width to the pattern on the phototool and normal to the conductor surface. Practically, however, either underdevelopment or overdevelopment of the dry film photoresist occurs. Underdevelopment produces a buildup of resist residue in the sidewall zone or developed channels that is sloped toward the adjacent sidewall, resulting in smaller spaces between the adjacent lines than is desired. When overdevelopment occurs the unexposed film resist edge is undercut, producing larger than desired spacing between adjacent lines. Additionally, there is the potential for some rounding at the top of the resist surface sidewall edges.
This inability to accurately reproduce the phototool throughout the thickness of the dry film resist affects the fine line resolution and reproduction characteristics of the reproduced circuit pattern. As circuit boards have become more complex and stacking of multiple boards has become prevalent, the need for higher density, finer resolution circuit patterns has evolved. Resolution has been viewed as the ability to reliably produce the smallest line and space between adjacent lines that can be reliably carried through the aforementioned five step processing. The thinness or smallness of the lines that can survive development and the narrowness of the gap or space between the adjacent lines in the circuit pattern have led to fine line resolution and reproduction standards in the printed circuit board industry which are used to define the desired density of the circuit board. The desired density is expressed in lined and space dimensions or a specific number of line pairs per millimeter. The fact that circuit boards consist of a nonporous or nonabsorbent substrate, such as metal, like copper, or a plastic, like the polyester film sold under the tradename of MYLAR, has made it difficult to apply the principles of xerography to effect the transfer of high resolution and high density images from a developed electrostatic latent image to a receiving surface, such as a circuit board. This nonporous and nonabsorbent receiving surface causes the image being transferred, especially when attempted with a liquid toner, to become distorted or "squished".
Also, it has been found with nonporous receiving substrates that both the photoconductor or electrostatically imageable surface and the receiving conductive surface must be stationary at the point of transfer of the toner image to achieve a transferred image of high resolution.
An additional problem is presented in transferring the developed latent image electrostatically to a nonabsorbent substrate, such as copper. The metal or copper surface forming the conductive receiving surface, as well as the electrostatically imageable surface, is uneven so that the spacing between the electrostatically imageable surface and the conductive receiving surface must be sufficient to avoid contact between the uneven surfaces of the photoconductor and the conductive receiving surface.
The use of a dry powder electrophotographic toner with a permanent master presents a further problem because of the normally soft surface of the photopolymer material used as the permanent master, especially in the unexposed areas. The dry powder toner will adhere to these softer areas and since they are the background areas which are not exposed, there is not even the cross-linking phenomenom present to somewhat harden the normally soft and sticky photopolymer. With repeated use, the dry powder toner builds up in the background areas, since it is difficult to clean off the master's soft surface prior to subsequent transfer. Alternately, the soft surface is worn away quickly during the cleaning attempts.
Attempts to prolong the useful life of a latent image or the substrate on which the latent image is formed have previously used a protective insulating layer of material over the photoconductor material. Representative processes employing this approach include the Canon NP process and the Katsuragawa process. These processes employed the use of a transparent protective insulating layer, but not in conjunction with a photopolymer that could be exposed to form a permanent master. These processes utilized polyethylene terephthalate as a solvent resistant layer on a cadmium sulfide drum or fine particles of cadmium sulfide in an elastomeric, resinous binder.
These problems are solved in the process of the present invention by providing a method of making a permanent latent image on a permanent master by using a protective overcoat material on the photopolymer master material, such as a liquid or a dry film resist. The permanent latent image on the photopolymer material is used to transfer a dry powder electrophotographically developed electrostatic latent image from the electrostatically imaged surface of the permanent master to a receiving surface. The receiving surface is preferably, but not exclusively, a conductive, nonabsorbent, and nonporous receiving surface of the type used to produce multiple printed circuit boards with a desired conductive pattern. Transfer can also be made to paper using this method.